Knowledge of tissue blood flow is important for effective hyperthermia treatment. Numerous investigators have suggested that blood flow in normal and tumor tissue changes during treatment and knowledge of the kinetics of these changes is important for accurate treatment planning. Tissue vasodilation during treatment is a complex process. The resultant blood flow during a fractionated treatment course changes from one treatment to another. As a result, the delineation of the underlying physiologic mechanisms responsible for these changes becomes increasingly important. This investigation will focus on establishing the kinetics of normal tissue blood flow in response to different fractionation schemes. Normal muscle tissue will be heated with either 45 degrees Centigrade or 43 degrees Centigrade hyperthermia for periods of up to one hour and blood flow in treated tissue will be measured during the treatment with different radioactively labeled microspheres. The response of tissue blood flow to different fractionation schemes will be evaluated for the two different tempatures. Previous data suggest that the transient tissue blood flow during four different hyperthermia fractions delivered at 72 hour intervals changes from treatment to treatment. This investigation will be an extension of the previous work and will examine blood flow changes during 24 hour and 168 hour fractionation intervals. To provide further elucidation of the underlying physiological mechanisms responsible for blood flow changes, different sizes of radioactively labeled microspheres will be employed during the different fractionation sequences. By comparison of blood flows measured with different sizes of radioactive microspheres in the same animal, an indication will be provided as to the changes in vessel diameter and response of the central mechanism during treatment. Correlation of these values to mathematical models will be preformed as they have been in the past. This will continue to allow measurement of tissue blood flow in spontaneous animal tumors treated with hyperthermia. A well developed network for accessing tumor bearing animals exists at the Medical College of Ohio and measurements of tumor and normal tissue blood flow under different fractionation schemes will be performed in vivo.